The invention relates to a method for rerouting a packet-mode data connection.
In principle, the method of the invention can be applied in any packet switched network but the invention is primarily intended for a FR network (Frame Relay network) for which its features (the use of buffering) are very well suited. However, the invention can also be used in a (generally faster) ATM (Asynchronous Transfer Mode) network, for example.
Replacing previous packet network connections, Frame Relay is a packet network technique for transmitting frames of varying lengths, a heavily stripped-down version of a nowadays generally used protocol (X.25) that requires a lot of processing. By stripping operations that have become in a way unnecessary with time because of improvements in the quality of the transmission network, frame relay in the Frame Relay technique has become faster and more efficient.
ATM is a new packet-switching technique in which the problems of conventional packet networks have been solved by introducing short packets of a standard length (53 bytes) known as cells. Each cell contains a payload part of 48 bytes and a header of 5 bytes in length.
However, in this connection the FR or ATM technology will not be described any further as the method of the invention is not connected to any FR or ATM specific arrangements, but the node devices of the network are only required to have an ability to buffer. Frame Relay service is generally described in the CCIT recommendation I.233, "Frame Mode Bearer Services", a the related protocol recommendation Q.922. For a more detailed description of the Frame Relay technique, a reference is made to the article "An Overview of Frame Relay Technology", Data-pro Management of Data Communications, McGraw-Hill Incorporated, April 1991. A closer description of the ATM technology can be found, for example, in the CCITT recommendation I.610, "B-ISDN operation and maintenance principles and functions", CCITT Study Group XVIII Geneva, Jun. 9-19, 1992, or in the recommendation I.361, "B-ISDN ATM Layer Specification", CCITT; ANSI T1.617 Annex D.
In packet networks, connections can be "protected" by having one or two alternative routes between nodes in addition to the main route. In this case, traffic can be transferred from one route to another if necessary. One of the requirements of the ATM network is, for example, that the order of the cells in the connection will remain invariable in the network. Because the ATM headers of the data cells do not have a sequence number to make the rearrangement of the cells possible, the connection has to travel via the same route through the network.
Transferring traffic from one route to another leads very probably to changes in the order of the cells and thus, loss of data, in case the delay of the new route is smaller than that of the old route.